Negative electrode active material and lithium ion secondary battery including negative electrode active material

ABSTRACT

The present disclosure provides a negative electrode active material that can realize excellent low temperature characteristics. The negative electrode active material for a lithium ion secondary battery disclosed herein includes a carbon material that is able to reversibly occlude and release lithium ions and a carbon coating layer that is formed on a surface of the carbon material, and the carbon coating layer contains carbon atoms and phosphorus atoms. In addition, in the carbon coating layer, when a peak of a P2p spectrum measured by X-ray photoelectron spectroscopy (XPS) is subjected to waveform separation, there is a peak at a position at which a binding energy is 131 eV, and an intensity ratio (ID/IG) of a peak intensity IG at 1,580 cm−1 to a peak intensity ID at 1,360 cm−1 in a Raman spectrum is 0.4 or more and 0.7 or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority based on Japanese Patent ApplicationNo. 2020-201681 filed on Dec. 4, 2020, and the entire contents of theapplication are incorporated herein by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to a negative electrode active materialand a lithium ion secondary battery including the negative electrodeactive material.

2. Description of the Background

In recent years, secondary batteries such as lithium ion secondarybatteries have been suitably used for portable power supplies forpersonal computers and mobile terminals and power supplies for drivingvehicles such as battery electric vehicles (BEV), hybrid electricvehicles (HEV), and plug-in hybrid electric vehicles (PHEV).

In the related art, a carbon material such as graphite is often used asa negative electrode active material for a lithium ion secondarybattery. Further, in order to improve battery performance, a negativeelectrode active material having a coating layer containing variouscompounds on a surface of a carbon material is known, and for example, anegative electrode active material having a coating layer containing aphosphorous-containing compound (for example, a phosphate compound,etc.) is disclosed (WO 2012/070153, Japanese Patent ApplicationPublication No. 2014-10998, WO 2018/173521, Japanese Patent ApplicationPublication No. 2011-29160, and Japanese Patent Application PublicationNo. 2018-181764).

SUMMARY

Incidentally, generally, in a low temperature environment (for example,in a −10° C. environment), the capacity of a lithium ion secondarybattery tends to decrease. Therefore, a lithium ion secondary batterythat can maintain a favorable capacity and has excellent low temperaturecharacteristics even in a low temperature environment is desired.

Therefore, the present disclosure has been made in view of the abovecircumstances, and a main object of the present disclosure is to providea negative electrode active material that can realize excellent lowtemperature characteristics. In addition, another object is to provide alithium ion secondary battery including such a negative electrode activematerial. In addition, another object is to provide a preferable methodof producing the negative electrode active material disclosed herein.

In order to address the above problems, the inventors conductedextensive studies and as a result, found that, in a negative electrodeactive material including a carbon material having on a surface thereofa coating layer having a structure in which phosphorus atoms and carbonatoms are bonded to each other, excellent low temperaturecharacteristics can be realized by adjusting the crystallinity of thecoating layer to be within a predetermined range.

That is, the negative electrode active material disclosed hereinincludes a carbon material that is able to reversibly occlude andrelease lithium ions and a carbon coating layer that is formed on asurface of the carbon material, and the carbon coating layer containscarbon atoms and phosphorus atoms. In addition, in the carbon coatinglayer, when a peak of a P2p spectrum measured by X-ray photoelectronspectroscopy (XPS) is subjected to waveform separation, it has a peak ata position at which a binding energy is 131 eV, and an intensity ratio(I_(D)/I_(G)) of a peak intensity I_(G) at 1,580 cm⁻¹ to a peakintensity I_(D) at 1,360 cm⁻¹ in a Raman spectrum is 0.4 or more and 0.7or less.

With such a configuration, it is possible to provide a negativeelectrode active material that imparts excellent low temperaturecharacteristics to a lithium ion secondary battery.

In addition, in a preferable aspect of the negative electrode activematerial disclosed herein, in the carbon coating layer, when a sum of apeak area of a C1s spectrum, a peak area of an O1s spectrum, and a peakarea of the P2p spectrum measured by XPS is 100%, a proportion of thepeak area at the position of 131 eV is 0.4% or more and 0.7% or less.

With such a configuration, it is possible to more suitably impartexcellent low temperature characteristics to a lithium ion secondarybattery.

In addition, in order to achieve the above object, a lithium ionsecondary battery including a negative electrode active materialdisclosed herein is provided. That is, the lithium ion secondary batterydisclosed herein includes a positive electrode, a negative electrode,and a non-aqueous electrolyte, the negative electrode has a negativeelectrode active material layer, and the negative electrode activematerial layer includes the negative electrode active material disclosedherein.

With such a configuration, it is possible to provide a lithium ionsecondary battery having excellent low temperature characteristics.

In addition, in order to achieve the above object, a preferable methodof producing the negative electrode active material disclosed herein isprovided. That is, the method of producing the negative electrode activematerial disclosed herein includes: preparing a carbon material that isable to reversibly occlude and release lithium ions; forming a carboncoating layer containing carbon atoms and phosphorus atoms on a surfaceof the carbon material by a CVD method; and firing the carbon materialhaving the carbon coating layer, and performing adjustment so that anintensity ratio (I_(D)I_(G)) of a peak intensity I_(G) at 1,580 cm⁻¹ toa peak intensity I_(D) at 1,360 cm⁻¹ in a Raman spectrum of the carboncoating layer is 0.4 or more and 0.7 or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a configurationof a lithium ion secondary battery according to one embodiment;

FIG. 2 is a schematic exploded view showing a configuration of a woundelectrode body of a lithium ion secondary battery according to oneembodiment; and

FIG. 3 is a schematic view showing a cross-sectional structure of oneparticle constituting a negative electrode active material according toone embodiment.

DETAILED DESCRIPTION

Hereinafter, one embodiment of technologies disclosed herein will bedescribed in detail with reference to the drawings. Here, componentsother than those specifically mentioned in this specification that arenecessary for implementing the present technologies can be recognized bythose skilled in the art as design matters based on the related art inthe field. The technology disclosed herein can be implemented based oncontent disclosed in this specification and common general technicalknowledge in the field. In addition, members and portions having thesame functions are denoted by the same reference number as in thefollowing drawings, and redundant descriptions thereof will be omittedor simplified. In addition, the sizes (a length, a width, a thickness,etc.) in the drawings do not reflect actual sizes.

The term “lithium ion secondary battery” in this specification refers toa secondary battery in which lithium ions are used as charge carriers,and charging and discharging are performed by movement of chargesaccording to lithium ions between a positive electrode and a negativeelectrode.

In addition, the terms “positive electrode active material” and“negative electrode active material” in this specification refer to asubstance that is able to reversibly occlude and release (typically,insert and desorb) chemical species (that is, lithium ions) as chargecarriers in a lithium ion secondary battery.

A lithium ion secondary battery 100 shown in FIG. 1 is a rectangularsealed battery constructed by accommodating a flat electrode body 20 anda non-aqueous electrolytic solution (not shown) inside a battery case30. The battery case 30 includes a positive electrode terminal 42 and anegative electrode terminal 44 for external connection. In addition, athin-walled safety valve 36 that is set to release an internal pressurewhen the internal pressure of the battery case 30 increases to apredetermined level or more is provided. In addition, in the batterycase 30, a liquid injection port (not shown) through which a non-aqueouselectrolytic solution is injected is provided. The material of thebattery case 30 is preferably a metal material having high strength,being lightweight, and having favorable thermal conductivity, andexamples of such a metal material include aluminum and steel.

As shown in FIG. 1 and FIG. 2, the electrode body 20 is a woundelectrode body in which a long sheet-shaped positive electrode 50 and along sheet-shaped the negative electrode 60 are laminated with two longsheet-shaped separators 70 therebetween, and wound around a windingaxis. The positive electrode 50 includes a positive electrode currentcollector 52 and a positive electrode active material layer 54 formed onone side or both sides of the positive electrode current collector 52 inthe longitudinal direction. On an edge on one side of the positiveelectrode current collector 52 in the winding axis direction (that is,the sheet width direction orthogonal to the longitudinal direction), apart in which the positive electrode active material layer 54 is notformed in a band shape along the edge and the positive electrode currentcollector 52 is exposed (that is, a positive electrode current collectorexposed part 52 a) is provided. In addition, the negative electrode 60includes a negative electrode current collector 62 and a negativeelectrode active material layer 64 formed on one side or both sides ofthe negative electrode current collector 62 in the longitudinaldirection. On an edge on the side opposite to one side of the negativeelectrode current collector 62 in the winding axis direction, a part inwhich the negative electrode active material layer 64 is not formed in aband shape along the edge and the negative electrode current collector62 is exposed (that is, a negative electrode current collector exposedpart 62 a) is provided. A positive electrode current collector plate 42a and a negative electrode current collector plate 44 a are bonded tothe positive electrode current collector exposed part 52 a and thenegative electrode current collector exposed part 62 a, respectively.The positive electrode current collector plate 42 a is electricallyconnected to the positive electrode terminal 42 for external connection,and realizes conduction between the inside and the outside of thebattery case 30. Similarly, the negative electrode current collectorplate 44 a is electrically connected to the negative electrode terminal44 for external connection, and realizes conduction between the insideand the outside of the battery case 30.

Regarding the positive electrode current collector 52 constituting thepositive electrode 50, for example, an aluminum foil may be exemplified.Regarding the positive electrode active material of the positiveelectrode active material layer 54, for example, lithium composite metaloxides having a layered structure, a spinel structure, or the like (forexample, LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNiO₂, LiCoO₂, LiFeO₂, LiMn₂O₄,LiNi_(0.5)Mn_(1.5)O₄, LiCrMnO₄, LiFePO₄, etc.) may be exemplified. Inaddition, the positive electrode active material layer 54 may contain aconductive material, a binder, and the like. Regarding the conductivematerial, for example, carbon black such as acetylene black (AB) orother carbon materials (graphite, etc.) can be suitably used. Regardingthe binder, for example, polyvinylidene fluoride (PVDF) or the like canbe used.

The positive electrode active material layer 54 can be formed bydispersing a positive electrode active material and a material (aconductive material, a binder, etc.) used as necessary in a suitablesolvent (for example, N-methyl-2-pyrrolidone: NMP) to prepare a paste(or slurry) composition, applying an appropriate amount of thecomposition to a surface of the positive electrode current collector 52,and drying it.

Regarding the negative electrode current collector 62 constituting thenegative electrode 60, for example, a copper foil may be exemplified.The negative electrode active material layer 64 contains a negativeelectrode active material disclosed herein. In addition, the negativeelectrode active material layer 64 may further contain a binder, athickener and the like. Regarding the binder, for example, styrenebutadiene rubber (SBR) or the like can be used. Regarding the thickener,for example, carboxymethyl cellulose (CMC) or the like can be used.

The negative electrode active material layer 64 can be formed bydispersing a negative electrode active material and a material (abinder, etc.) used as necessary in a suitable solvent (for example,deionized water) to prepare a paste (or slurry) composition, applying anappropriate amount of the composition to a surface of the negativeelectrode current collector 62, and drying it.

Regarding the separator 70, various microporous sheets similar to thoseused in lithium ion secondary batteries in the related art can be used,and for example, a microporous resin sheet made of a resin such aspolyethylene (PE) and polypropylene (PP) may be exemplified. Such amicroporous resin sheet may have a single-layer structure or amulti-layer structure including two or more layers (for example, athree-layer structure in which a PP layer is laminated on both surfacesof a PE layer). In addition, on the surface of the separator 70, a heatresistant layer (HRL) may be provided, and for example, a ceramic(alumina, boehmite, etc.) may be applied.

Regarding the non-aqueous electrolyte, those used in the lithium ionsecondary batteries in the related art can be used, and typically anorganic solvent (non-aqueous solvent) containing a supporting salt canbe used. Regarding the non-aqueous solvent, aprotic solvents such ascarbonates, esters, and ethers can be used. Among these, carbonates, forexample, ethylene carbonate (EC), diethyl carbonate (DEC), dimethylcarbonate (DMC), ethyl methyl carbonate (EMC), and the like can besuitably used. Alternatively, fluorine solvents such as fluorinatedcarbonates, for example, monofluoroethylene carbonate (MFEC),difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethylcarbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC) can bepreferably used. These non-aqueous solvents can be used alone or two ormore thereof can be used in appropriate combination. Regarding thesupporting salt, lithium salts, for example, LiPF₆, LiBF₄, and LiClO₄,can be suitably used. The concentration of the supporting salt is notparticularly limited, and is preferably about 0.7 mol/L or more and 1.3mol/L or less.

Here, the non-aqueous electrolyte may contain components other than theabove non-aqueous solvent and supporting salt as long as the effects ofthe technology disclosed herein are not significantly impaired, and maycontain various additives, for example, a gas generating agent, a filmforming agent, a dispersant, and a thickener.

FIG. 3 schematically shows a cross section of a particle (a negativeelectrode active material particle 80) constituting the negativeelectrode active material disclosed herein. The negative electrodeactive material particle 80 includes a carbon material 82 and a carboncoating layer 84 that covers the surface of the carbon material 82.

The carbon material 82 may be a material (typically, particulate)composed of carbon atoms that is able to reversibly occlude and releaselithium ions. Regarding the carbon material 82, for example, aparticulate carbon material (carbon particle) having a graphitestructure (layered structure) in at least a part may be exemplified. Inaddition, a so-called graphite material (graphite), a non-graphitizablecarbon material (hard carbon), an easily graphitizable carbon material(soft carbon), and a material having a structure combining these can beused as carbon materials. Among these, graphite particles such asnatural graphite can be preferably used.

The carbon coating layer 84 may be formed on at least a part of thesurface of the carbon material 82, and the carbon coating layer 84 isformed on preferably 70% or more, more preferably 80% or more, and stillmore preferably 90% or more (or 100%) of the surface area of the carbonmaterial 82. When the carbon coating layer 84 is formed at a high ratio,since the area of the carbon coating layer 84 in contact with anon-aqueous electrolytic solution increases, the effect of improving lowtemperature characteristics obtained by the carbon coating layer 84 canbe exhibited at a higher level.

The average particle size of the negative electrode active materialparticle 80 is not particularly limited, and in consideration of handingproperties, ease of forming the carbon coating layer 84, and the like,it may be generally 0.5 μm or more and 50 μm or less, and typically 1 μmor more and 20 μm or less, for example, 5 μm or more and 10 μm or less.Here, the “average particle size” in this specification is a particlesize of cumulative 50% in a volume-based particle size distributionobtained by measuring a particle size distribution based on a laserdiffraction/light scattering method.

The average thickness of the carbon coating layer 84 is not particularlylimited, and is generally 2 nm or more and 2 μm or less, and typically 5nm or more and 1 μm or less.

The carbon coating layer 84 includes carbon atoms and phosphorus atoms,and may further contain one or two or more other elements such as oxygenatoms. The element ratio of the carbon coating layer 84 and abundanceproportions of bonding forms between atoms can be measured through X-rayphotoelectron spectroscopy (XPS). XPS is a method in which X-rays areemitted to a surface of a sample, the emitted photoelectron energy ismeasured, and elements constituting the surface of the sample and theirelectron states are analyzed. Since the spectrum obtained by XPSexhibits a substance-specific pattern and a peak area proportional tothe amount of a substance, it is possible to qualitatively andquantitatively analyze the substances. Therefore, it is possible todetermine elements constituting the carbon coating layer 84 present onthe surface of the negative electrode active material particle 80 andbonding forms between atoms.

In the negative electrode active material disclosed herein, since thecarbon coating layer 84 may contain carbon atoms, phosphorus atoms, andoxygen atoms, the peak of the Cis spectrum measured through the XPS(typically, a peak with a peak top in a binding energy range of 279 eVto 298 eV), a peak area of the O1s spectrum (typically, a peak with apeak top in a binding energy range of 528 eV to 540 eV), and a peak areaof the P2p spectrum (typically, a peak with a peak top in a bindingenergy range of 128 eV to 140 eV) can be measured. Here, the peak of theC1s spectrum is a peak derived from the energy of the is orbital of thecarbon atom, the peak of the 01 spectrum is a peak derived from theenergy of the Is orbital of the oxygen atom, and the peak of the P2pspectrum is a peak derived from the energy of the 2 p orbital of thephosphorus atom.

When the total sum (hereinafter referred to as a “sum T of peak areas”)of the peak area of the C1s spectrum, the peak area of the O1s spectrum,and the peak area of the P2p spectrum is set as 100%, the peak area ofthe C1s spectrum can be typically 80% or more (for example, 85% ormore), the peak area of the O1s spectrum can be 20% or less (typically,15% or less), and the peak area of the P2p spectrum can be 5% or less(typically 3% or less). The peak area of the P2p spectrum is preferably2% or less (for example, 1.9% or less). In addition, the proportion ofthe peak area of the P2p spectrum is typically 0.1% or more, preferably0.8% or more, and more preferably 1.4% or more. With such proportions,particularly excellent low temperature characteristics can be realized.

Since the peak of the P2p spectrum can be composed of a plurality ofadjacent peaks overlapping, waveform separation is performed by curvefitting (typically, fitting based on a nonlinear least squares method),and for example, waveforms can be separated into a peak at a positionwith a binding energy of 131 eV, a peak at a position with a bindingenergy of 133 eV, and a peak at a position with a binding energy of 135eV. Here, waveform separation can be performed using, for example,software “MultiPak” (commercially available from ULVAC-PHI Inc).Typically, the peak at the position of 131 eV is a peak derived from theC—P—C bond, the peak at the position of 133 eV is a peak derived fromthe P—O bond, and the peak at the position of 135 eV is a peak derivedfrom the O—P—O bond. Here, the “peak at the position of 131 eV” in thisspecification includes a deviation of the position of the peak top thatmay occur due to measurement conditions and the like, and may include apeak at a position in the vicinity of 131 eV. That is, typically, it maybe a peak at a position of 131 eV±0.9 eV, for example, a peak at aposition of 131 eV±0.5 eV or 131 eV±0.1 eV. The same applies to the“peak at the position of 133 eV” and the “peak at the position of 135eV,” which can be a peak at a position of 133 eV±0.9 eV (for example,133 eV±0.5 eV and 133 eV±0.1 eV) and a peak at a position of 135 eV±0.9eV (for example, 135 eV±0.5 eV and 135 eV±0.1 eV), respectively.

The carbon coating layer 84 preferably has a peak at the position of 131eV. That is, the carbon coating layer 84 preferably has a C—P—C bond.Therefore, a desolvation reaction of lithium ions can be promoted due tosurplus electrons derived from phosphorus atoms so that low temperaturecharacteristics can be improved. Here, “having a peak” means that thereis a peak having a peak area of 0.1% or more when the sum T of the peakareas is set as 100%.

In addition, the proportion of the peak area at the position of 131 eVwhen the sum T of the peak areas is set as 100% is not particularlylimited, and is, for example, 0.1% or more and 5% or less, preferably0.4% or more and 0.7% or less, and more preferably 0.6% or more and 0.7%or less. Within such a range, since a desolvation reaction of lithiumions can be promoted due to surplus electrons derived from phosphorusatoms, low temperature characteristics can be improved.

The proportion of the peak area at the position of 133 eV when the sum Tof the peak areas is set as 100% is not particularly limited, and it canbe, for example, 0.1% or more and 2% or less (for example, 0.1% or moreand 0.5% or less), and is preferably 0.3% or more and 0.5% or less.

The proportion of the peak area at the position of 135 eV when the sum Tof the peak areas is set as 100% is not particularly limited, and it canbe, for example, 0.1% or more and 2% or less (for example, 0.2% or moreand 0.5% or less), and is preferably 0.3% or more and 0.5% or less.

The carbon coating layer 84 has an amorphous structure, and may containvery small crystallites composed of carbon atoms in sp² hybridizedorbitals, carbon atoms having a bond formation other than through sp²hybridized orbitals, and the like. The crystallinity of the carboncoating layer 84 is evaluated by Raman spectrum analysis. In order tomeasure the Raman spectrum, a conventionally known method can beappropriately used. In the carbon coating layer 84, in Raman spectrumanalysis using an appropriate laser light (for example, an argon ionlaser) as a light source, when the peak intensity at 1,580 cm⁻¹ isrepresented as I_(G) and the peak intensity at 1,360 cm⁻¹ is representedas I_(D), the value of the intensity ratio (I_(D)/I_(G)) of I_(D) toI_(G) is preferably 0.4 or more and 0.7 or less and more preferably 0.6or more and 0.7 or less. Within such a range, the capacity of acceptinglithium ions at low temperatures can be improved, and low temperaturecharacteristics can be improved. Here, in this specification, the “peakintensity at 1,580 cm⁻¹” is a peak intensity in the vicinity of 1,580cm⁻¹ (for example, a range (G band) of 1,570 cm⁻¹ to 1,620 cm⁻¹). Inaddition, in this specification, the “peak intensity at 1,360 cm⁻¹” is apeak intensity in the vicinity of 1,360 cm⁻¹ (for example, in a range (Dband) of 1,300 cm⁻¹ to 1,400 cm⁻¹).

Next, a preferable method of producing the negative electrode activematerial disclosed herein will be described. Here, the method ofproducing the negative electrode active material disclosed herein is notlimited to the following.

The preferable method of producing the negative electrode activematerial disclosed herein includes a process of preparing a carbonmaterial that is able to reversibly occlude and release lithium ions(hereinafter referred to as a “preparation process”), a process offorming a carbon coating layer containing carbon atoms and phosphorusatoms on the surface of the carbon material by a CVD method (hereinafterreferred to as a “coating process”), and a process of firing the carbonmaterial having the carbon coating layer (hereinafter referred to as a“firing process”).

First, the preparation process will be described. As the carbon materialthat is able to reversibly occlude and release lithium ions, a materialthat can be used for the above carbon material 82 may be used. Such amaterial may be purchased as a commercial product or may be produced bya conventionally known method.

Next, the coating process will be described. A chemical vapor deposition(CVD) method can be suitably used to form a carbon coating layer on thesurface of the prepared carbon material. According to the CVD method, aC—P—C bond can be suitably formed on the carbon coating layer. Here, theCVD method is generally roughly classified into a thermal CVD method, aplasma CVD method, an optical CVD method and the like, but any methodcan be used, and here, the thermal CVD method will be described as anexample. Here, in this specification, “1 sccm” is a unit indicating aflow rate at which 1 cc (1 mL) is supplied per minute under anatmospheric pressure at 0° C.

Regarding the thermal CVD method itself, the same process as in therelated art may be used, and no special device is required. For example,first, a carbon material is placed in a reaction container (for example,tubular furnace) as a coating object. Next, the inside of the reactioncontainer is purged with an inert gas (for example, a gas that does notcontribute to the reaction of carbon coating layer formation such as Argas). After purging with the inert gas, the temperature in the reactioncontainer is raised (for example, 800° C. or higher and 1,000° C. orlower). Then, a precursor gas of a carbon coating is supplied into thereaction container together with the inert gas. A supply rate (flowrate) of the inert gas is not particularly limited, and can be, forexample, 50 sccm or more and 350 sccm or less. In addition, the supplyrate (flow rate) of the precursor gas is not particularly limited, andcan be, for example, 100 sccm or more and 300 sccm or less. In addition,a supply amount (flow rate) of the precursor gas and the inert gas canbe provided, for example, at a ratio of 1:3 to 3:2. The reaction timeafter such supply is not particularly limited because it can be changeddepending on a desired thickness of the carbon coating layer, and canbe, for example, a time of about 45 minutes to 90 minutes. In addition,during such a reaction, it is preferable to rotate the reactioncontainer, and for example, the reaction container can be rotated at arotational speed of 10 rpm or more and 50 rpm or less. According to suchrotation, a more uniform carbon coating layer can be formed over theentire surface of the carbon material. Here, these operations can beperformed in the reaction container under atmospheric pressure or in areduced pressure state (for example, 1×10⁴ Pa or more and 8×10⁴ Pa orless).

As a precursor that serves as a carbon atom supply source, a hydrocarbongas used in the conventional CVD method can be used, and examplesthereof include methane, ethylene, and acetylene. These may be used incombination of one or two or more thereof. Among these, acetylene can bepreferably used.

A precursor that serves as a phosphorus atom supply source is notparticularly limited, and for example, phosphorus oxychloride (chemicalformula: POCl₃) can be preferably used. Since phosphorus oxychloride isa liquid at mom temperature under atmospheric pressure, it can be usedas a precursor gas for forming a carbon coating by gasifying it byheating and then mixing it with a hydrocarbon gas.

The mixing ratio of hydrocarbon gas and phosphorus oxychloride gas isnot particularly limited because it can be adjusted according to theamount of phosphorus atoms to be introduced into the carbon coatinglayer, and for example, a mixture of hydrocarbon gas and phosphorusoxychloride in a molar ratio range of 20:1 to 3:1 can be used.

Next, the firing process will be described. In the firing process, thecarbon material having the carbon coating layer produced in the coatingprocess is fired under an inert gas atmosphere (for example, Ar gas),and thus the crystallinity of the graphite structure having conductivitycan be improved. That is, in the Raman spectrum of the carbon coatinglayer, the intensity ratio I_(D)/I_(G) between the peak intensity I_(G)at 1,580 cm⁻¹ and the peak intensity I_(D) at 1,360 cm⁻¹ can be reduced,and can be adjusted to a desired intensity ratio I_(D)/I_(G) (forexample, 0.4 or more and 0.7 or less). The firing temperature and thefiring time are not particularly limited, and for example, it ispreferable to perform firing at a firing temperature of 900° C. to1,100° C. (for example, 1,000° C.) for 1 hour to 9 hours (for example, 2hours to 8 hours). Here, if the firing temperature is too high or thefiring time is too long, there is a risk of phosphorus atoms and oxygenatoms contained in the carbon coating layer undergoing a dereaction.

One example of the preferable method of producing the negative electrodeactive material disclosed herein has been described above. The lithiumion secondary battery 100 including such a negative electrode activematerial can be used for various applications. For example, it can besuitably used as a high output power source for a motor (driving powersupply) mounted in a vehicle. The type of vehicle is not particularlylimited, and typically automobiles, for example, plug-in hybrid electricvehicles (PHEV), hybrid electric vehicles (HEV), and battery electricvehicles (BEV) may be exemplified. The lithium ion secondary battery 100can be used in the form of an assembled battery in which a plurality ofbatteries are electrically connected.

Examples related to the technology disclosed herein will be describedbelow, and are not intended to limit the technology disclosed herein tosuch examples.

Production of Negative Electrode Active Material

A carbon coating layer was formed on the surface of graphite (SG-BH8,commercially available from Ito Graphite Co., Ltd.) using a rotary CVDmethod. Here, a rotary tubular atmospheric furnace (commerciallyavailable from HeatTec Co., Ltd.) was used as the rotary CVD device.Table 1 shows conditions for the CVD method in Examples 1 to 6.

Example 1

20 g of graphite (SG-BH8) was accommodated in a tubular furnace, and theinside of the tubular furnace was purged with Ar gas. After thetemperature inside the tubular furnace was set to 800° C., whilerotating the tubular furnace at a rate of 10 rpm, a mixed gas in whichC₂H₂ gas and POCl₃ gas as precursor gases were mixed at a molar ratio of10:1 was supplied at a flow rate of 200 sccm for 60 minutes, and acarbon coating layer was formed. Here, during this operation, Ar gas wasconstantly supplied at a flow rate of 300 sccm.

Example 2

20 g of graphite (SG-BH8) was accommodated in a tubular furnace, and theinside of the tubular furnace was purged with Ar gas. After thetemperature in the tubular furnace was set to 950° C., while rotatingthe tubular furnace at a rate of 10 rpm, CH₄ gas as a precursor gas wassupplied at a flow rate of 150 sccm for 80 minutes, and a carbon coatinglayer was formed. Here, during this operation, Ar gas was constantlysupplied at a flow rate of 50 sccm. Then, additional firing wasperformed at 1,000° C. for 2 hours in an Ar gas atmosphere.

Example 3

A negative electrode active material was produced in the same manner asin Example 1, and additional firing was then performed on the negativeelectrode active material at 1,000° C. for 2 hours in an Ar atmosphere.

Example 4

Example 4 was performed in the same manner as in Example 3 except thatadditional firing in Example 3 was performed for 4 hours.

Example 5

Example 5 was performed in the same manner as in Example 3 except thatadditional firing in Example 3 was performed for 8 hours.

Example 6

Example 6 was performed in the same manner as in Example 3 except thatadditional firing in Example 3 was performed for 10 hours.

Example 7

50 g of the negative electrode active material produced by the method ofExample 2 was prepared, and the surface of the negative electrode activematerial was processed by barrel sputtering using Li₃PO₄. Here, thesputtering rate was set to 0.01 g/h, and sputtering was performed for 3hours.

TABLE 1 Flow rate of Flow rate Additional Temperature Time Precursorprecursor gas of Ar gas firing time (° C.) (minutes) gas (sccm) (sccm)(hour) Example 1 800 60 C₂H₂/POCl₃ 200 300 — Example 2 950 80 CH₄ 150 502 Example 3 800 60 C₂H₂/POCl₃ 200 300 2 Example 4 800 60 C₂H₂/POCl₃ 200300 4 Example 5 800 60 C₂H₂/POCl₃ 200 300 8 Example 6 800 60 C₂H₂/POCl₃200 300 10

Composition Analysis of Surface of Negative Electrode Active Material

The surfaces (carbon coating layer) of the produced negative electrodeactive materials were measured through X-ray photoelectron spectroscopy(XPS). Specific measurement conditions were as follows.

Measurement device: scanning X-ray photoelectron spectrometer (u-XPS)QuanteraII(commercially available from ULVAC-PHI Inc)X-ray source used: mono-A1Ka line (1,486.6 V)Photoelectron take-off angle: 35°X-ray beam diameter: about 100 μmNeutralization gun conditions: 1.0 V, 20 μA

A peak area Ac of the C1s spectrum, a peak area Ao of the O1s spectrumand a peak area Ap of the P2p spectrum obtained by the XPS werecalculated. Table 2 shows a proportion of Ac (the column C1s in Table2), a proportion of Ao (the column O1s in Table 2), and a proportion ofAp (the column P2p in Table 2) when a sum of these Ac, Ao, and Ap wasset as 100%.

In addition, the peak of the P2p spectrum was subjected to waveformseparation using software “MultiPak” (commercially available fromULVAC-PHI Inc), and separated into peaks at positions of 131 eV, 133 eV,and 135 eV. Then, Table 2 shows proportions of respective peak areaswith respect to a total of the Ac+Ao+Ap.

The produced negative electrode active materials were analyzed by aRaman spectropic analyzer (commercially available from Renishaw,microscopic Raman spectroscopic measurement device, inVia Reflex 785S,an excitation wavelength of 532 nm, Ar laser) to obtain a Ramanspectrum. The peak intensity at 1,580 cm⁻¹ of the Raman spectrum wasmeasured as I_(G), the peak intensity at 1,360 cm⁻¹ was measured asI_(D), and the intensity ratio (I_(D)/I_(G)) of I_(D) to I_(G) wasobtained. The values thereof are shown in Table 2.

Construction of Lithium Ion Secondary Battery for Evaluation

A lithium nickel cobalt manganese composite oxide(LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, hereinafter referred to as “NCM”) as apositive electrode active material, acetylene black (AB) as a conductivematerial, and polyvinylidene fluoride (PVdF) as a binder were mixed inN-methyl-2pyrrolidone so that the mass ratio of NCM:AB:PVdF was 92:5:3,and thereby a paste for forming a positive electrode active materiallayer was prepared. This paste was applied to an aluminum foil currentcollector with a thickness of 15 μm, dried and then pressed to produce asheet-shaped positive electrode.

The produced negative electrode active material (C), styrene butadienerubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as athickener were mixed in deionized water so that the mass ratio ofC:SBR:CMC was 99:0.5:0.5, and thereby a paste for forming a negativeelectrode active material layer was prepared. This paste was applied toa copper foil current collector with a thickness of 10 μm, dried andthen pressed to produce a sheet-shaped negative electrode.

In addition, two porous polyolefin sheets having a three-layer structureof PP/PE/PP as a separator and having a thickness of 24 μm wereprepared. An HRL including alumina and boehmite and having a thicknessof 4 μm was formed on the surface of the separator facing the positiveelectrode.

A wound electrode body was produced by laminating and winding theproduced sheet-shaped positive electrode and negative electrode so thatthey faced each other with a separator therebetween. A current collectorplate bonded to an electrode terminal was bonded to the wound electrodebody, which was accommodated in a battery case. Then, a non-aqueouselectrolytic solution was injected from a liquid injection port of thebattery case and sealed. As the non-aqueous electrolytic solution, asolution in which LiPF₆ as a supporting salt was dissolved at aconcentration of 1.0 mol/L in a mixed solvent containing ethylenecarbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate(EMC) at a volume ratio of 3:3:4 was used. As described above, a lithiumion secondary battery for evaluation was obtained.

Activation Treatment

The produced lithium ion secondary battery for evaluation was placed ina 25° C. environment. A constant current-constant voltage method wasused, each lithium ion secondary battery for evaluation was charged to4.1 V with a constant current at a current value of ⅓C, constant voltagecharging was then performed until the current value became 1/50C, andthe battery was fully charged. Then, each lithium ion secondary batteryfor evaluation was discharged to 3.0 V with a constant current at acurrent value of ⅓C. Here, “1C” indicates a magnitude of a current atwhich a battery can be charged from a state of charge (SOC) of 0% to100% in one hour.

Evaluation of −10° C. Acceptability

Each lithium ion secondary battery for evaluation that was subjected tothe above activation treatment was placed in a −10° C. environment. Eachlithium ion secondary battery for evaluation was set to a SOC of 0%,charged with a constant current at a current value of 5C, and the timerequired for the SOC to reach 100% was measured. Table 2 shows therelative value of the time in each example when the time in Example 1was set as 1.00 as “10° C. acceptability.” Here, if the time is longer,the capacity of accepting Li in a −10° C. environment is larger.Therefore, a larger numerical value of −10° C. acceptability indicatesbetter battery performance.

TABLE 2 −10° C. C1s (%) O1s (%) P2p (%) 131 eV (%) 133 eV (%) 135 eV (%)I_(D)/I_(G) acceptability Example 1 93.3 5.4 1.3 0.4 0.2 0.7 0.9 1.00Example 2 98.8 1.2 0 0 0 0 0.5 0.95 Example 3 87 11 1.9 0.7 0.5 0.5 0.71.12 Example 4 86 13 1.4 0.6 0.3 0.3 0.6 1.10 Example 5 91 8.4 0.8 0.40.1 0.2 0.4 1.05 Example 6 94.2 5.7 0.1 0.1 0 0 0.3 0.98 Example 7 90.67.2 2.2 0 2.1 0.1 0.6 1.02

As shown in Table 2, it was confirmed that Examples 3 to 5 in which,when the P2p spectrum measured through XPS was subjected to waveformseparation, there was a peak at the position of 131 eV and the value ofI_(D)/I_(G) was 0.4 or more and 0.7 or less had better −10° C.acceptability than Example 1 and Example 6 in which there was a peak atthe position of 131 eV, but the value of I_(D)/I_(G) was outside therange of 0.4 or more and 0.7 or less. In addition, in Example 7, even ifthe value of I_(D)/I_(G) was 0.6, since it did not have a peak at theposition of 131 eV, the −10° C. acceptability was not better than inExamples 3 to 5.

In addition, it could be understood that, since Example 3 and Example 4exhibited particularly excellent −10° C. acceptability, the value ofI_(D)/I_(G) is more preferably 0.6 or more and 0.7 or less.

While specific examples of the technology disclosed herein have beendescribed above in detail, these are only examples, and do not limit thescope of the claims. The technologies described in the scope of theclaims include various modifications and alternations of the specificexamples exemplified above.

What is claimed is:
 1. A negative electrode active material for a lithium ion secondary battery, comprising: a carbon material that is able to reversibly occlude and release lithium ions; and a carbon coating layer that is formed on a surface of the carbon material, wherein the carbon coating layer includes carbon atoms and phosphorus atoms, and in the carbon coating layer, when a peak of a P2p spectrum measured by X-ray photoelectron spectroscopy (XPS) is subjected to waveform separation, there is a peak at a position at which a binding energy is 131 eV, and an intensity ratio (I_(D)/I_(G)) of a peak intensity I_(G) at 1,580 cm⁻¹ to a peak intensity I_(D) at 1,360 cm⁻¹ in a Raman spectrum is 0.4 or more and 0.7 or less.
 2. The negative electrode active material according to claim 1, wherein, in the carbon coating layer, when a sum of a peak area of a C1s spectrum, a peak area of an O1s spectrum and a peak area of the P2p spectrum measured by XPS is 100%, a proportion of the peak area at the position of 131 eV is 0.4% or more and 0.7% or less.
 3. A lithium ion secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode active material layer, and the negative electrode active material layer includes the negative electrode active material according to claim
 1. 4. A method of producing a negative electrode active material for a lithium ion secondary battery, the method comprising: preparing a carbon material that is able to reversibly occlude and release lithium ions; forming a carbon coating layer containing carbon atoms and phosphorus atoms on a surface of the carbon material by a CVD method; and firing the carbon material having the carbon coating layer, and performing adjustment so that an intensity ratio (I_(D)/I_(G)) of a peak intensity I_(G) at 1,580 cm⁻¹ to a peak intensity I_(D) at 1,360 cm⁻¹ in a Raman spectrum of the carbon coating layer is 0.4 or more and 0.7 or less. 